Such a manipulator is described in U.S. Pat. No. 6,388,262.
The known manipulator is used to position and orient a component in the form of a specimen in a Transmission Electron Microscope (TEM).
The manipulator comprises a support arm housing, which sticks through the housing of the TEM on which the manipulator is mounted. One end of the support arm housing is thereby located inside the apparatus, and thus surrounded by vacuum, while the other end is exposed to atmosphere. To avoid leakage the support arm housing shows an O-ring seal where it passes through the housing of the TEM, thereby sealing the interior of the TEM from atmosphere.
The support arm housing shows a bore from end to end. In this bore a specimen holder is mounted. The specimen holder comprises a rod with a ball joint located between its ends. One end is located inside the vacuum of the TEM, while the other side is exposed to atmosphere. The ball joint closely fits the bore of the support arm housing, and shows an O-ring seal, thereby sealing the interior of the TEM from atmosphere. The ball joint allows the rod to move with respect to the support arm housing with a number of degrees of freedom. The end of the rod inside the TEM is equipped to hold a TEM specimen grid with a specimen placed thereon, while the other end is connected to motorized drives. Moving the distal end of the rod located in vacuum in an X-Y plane (perpendicular to the particle-optical axis of the TEM) results in a translation of the specimen holder in that plane, while a rotation of the rod along its axis is translated in a rotation of the specimen holder in said plane. Hereby it is possible to position and orient the specimen, located inside the TEM, from the outside with the motorized drives.
As known to the person skilled in the art a TEM employs a beam of particles in the form of electrons with an energy of typically between 80-300 keV travelling along a particle-optical axis. The beam of electrons is, for example, focused by particle-optical lenses and, for example, deflected by particle-optical deflectors.
A thin specimen, also known as a sample, with a thickness of typically between 50 and 100 nm is placed on the particle-optical axis and is irradiated by the electrons. Some of the electrons pass through the specimen and are imaged on a detector.
Commercially available TEM's may nowadays achieve a spatial resolution of less than 0.1 manometers.
Such a resolution is only achievable when during the time needed to acquire an image, the specimen is stationary with respect to the particle-optical axis within 0.1 nm. Any movement of the specimen with respect to the particle-optical axis, e.g. due to thermal drift or due to e.g. acoustic vibrations of the manipulator holding the specimen, deteriorates the image. Acquiring an image takes typically between several seconds to several minutes. The requirements for the stability of the manipulator with respect to the particle-optical axis are thus extremely rigid.
When using extreme magnifications the field of view (FoV) is extremely small, e.g. 50·50 nm2. It is a challenge to avoid a jerky movement of the image when positioning the specimen at this magnification. Not only must the smallest displacement of the specimen be very small; the movements must also be performed very smooth. Furthermore any backlash or after-effects of movement, such as residual movement or vibration, must be minimal.
The resolution of electron microscopes is rapidly improving due to the introduction of particle-optical correctors correcting spherical and/or chromatic aberrations of one or more of the particle-optical lenses. Commercial instruments are available that achieve resolutions below 0.05 nm under optimum conditions. As a result the demand on the mechanical stability of the manipulator is becoming more severe.
A problem of the manipulator described in U.S. Pat. No. 6,388,262 is that is has a multitude of parts and couplings that must be very precisely machined. As a result such a manipulator is expensive. Also, due to the number of elements producing friction and/or play, such as the O-ring seals, it is difficult to construct such a manipulator with the required stability and smoothness of motion while lacking after-effects. The friction may introduce a jerky movement, as the motors must overcome the friction when making a small step, resulting in slip/stick effects. The play between parts may result in inaccuracy of the positioning.
A related problem is that the length of the rod, the number of couplings and the O-ring seals result in a soft coupling between the housing of the apparatus (and thus the particle-optical axis) and the end where the specimen resides. As known to the person skilled in the art, a stiff coupling (as opposed to a soft coupling) between elements is preferred when counteracting the effects of vibrations.
Another problem of the aforementioned manipulator is that part of the manipulator sticks out of the apparatus. Variations in air pressure outside the instrument, due to climate control, ambient temperature changes, etc., may thereby cause positional changes of the attachment mechanism inside the vacuum, and thus undesired movement of the specimen.